Static limit switch for power press or the like



1966 F. E. HEIBERGER 3,227,897

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Jan- 4, 1965 F. E. HEIBERGER 3,227,897

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'IIIIJIIIIIIIIIIIIIIII United States Patent 3,227,897 STATIC LIMIT SWITCH FOR POWER PRESS OR THE LIKE Francis E. Heiberger, Elmhurst, 111., assignor to Danly Machine Specialties, Inc., Chicago, Ill., a corporation of Illinois Filed Aug. 30, 1962, Ser. No. 220,458 8 Claims. (Cl. 307-116) The present invention relates to power presses and more particularly to control means for producing an output voltage over predetermined angular ranges of the press crankshaft. I

A typical power press employs switches operated by the crankshaft and referred to in the field as limit switches for the control of the clutch and brake of the press. One of these switches is referred to as the run limit switch and the other as the anti-repeat limit switch. The run limit switch is for the purpose of insuring that the operator will keep his hands on the dual pushbuttons forming a part of the press control from the time that the slide begins its movement until the slide has substantially completed its downward stroke, after which the slide is in a safe position and the operator is free to take his hands off. The run limit switch is generally phased to close at an angle of about 170 of crankshaft rotation and to open again just prior to the time that the slide reaches its rest position at the top of the stroke. The anti-repeat pushbutton and its associated control circuitry is for the purpose of insuring that the operator will not cause the press to recycle simply by keeping his hands on the pushbuttons; that is, the operators hands must be released and reapplied before another cycle of the press can be initiated. This requires opening of one of the control circuits for a short space of time, say, over an arc of about as the slide completes the upper portion of its stroke. For details of logic circuitry intended to be controlled by the limit switches, reference is made to my copending application Ser. 169,527 filed January 29, 1962.

Since the safety of the operator depends directly upon the reliability of the timed making and breaking of the above mentioned circuits, efforts have been made in the past to develop strong and wear resistant cam switches capable of giving reliable service over the life of the press. However, an increase in the massiveness and complexity of the switches has not been accompanied by a corresponding increase in reliability and, because of inertia, the development of high impact forces, and the like, efforts to develop improved switches of the mechanical type have been largely self-defeating.

Accordingly it is an object of the present invention to provide a circuit control arrangement for a power press capable of operating the circuits formerly controlled by run and anti-repeat switches but which avoids the necessity for physical switching at the press crankshaft. It is a related object to provide an improved press control arrangement which is static that is, which does not have any engaged moving parts or wear surfaces. More specifically, it is an object to provide a press control arrangement in which the switch operating cam generally used at the end of the crankshaft is replaced by a disc or the like of magnetic material cooperating with fixed but preferably adjustable pickup heads which magnetically couple to the disc through an air gap.

It is another object of the present invention to provide a press control arrangement achieving almost perfect safety and reliability and which is so designed that failure of any component therein causes the'device to fail safe with the result that no output signal is produced so that the press is safely and automatically brought to a stop.

It is a more detailed object of the invention to provide a control arrangement which employs a proximity trans- 3,227,897 Patented Jan. 4, 1966 ducer to control the anti-repeat function and which permits starting the press from a dead stop and velocity transducer to control the run function and which responds to the velocity of the crankshaft over a predetermined range and which is therefore independent of any foreign agent such as a piece of magnetic scrap metal which might inadvertently get into the neighborhood of the velocity transducer to affect its output.

It is still another object of the present invention to provide novel pickup control circuitry which, in spite of the use of pickups which must act through an air gap is nevertheless accurate and precise, initiating a signal and producing cut-off of such signal, analogously to the making and breaking of the conventional cam switch, consistently at a predetermined phase position of the crankshaft. It is an object of the invention in one of its aspects to produce a control arrangement which employs magnetic devices but which is capable of operating over a wide range of press speed and which avoids the phase shifting effects usually encountered in such devices.

Finally, it is an object of the present invention to provide a press control arrangement which does not require any critical circuit adjustments, which may be operated successfully over a wide range of air gap, and in which consistent effects are achieved over many thousands of cycles free of the effects of age, temperature, vibration, or other environmental conditions. Finally it is an object of the present invention to provide a press control arrange ment which may be inexpensively constructed utilizing non-critical and readily available components and which may be conveniently constructed in the form of compact plug-in modules mounted in a control box directly fixed to the press frame.

Other objects and advantages of the invention will be apparent upon reading the attached detailed description and upon reference to the drawings in which:

FIGURE 1 is an end view, with the front portion of the housing being removed, of a disc and pickup assembly constructed in accordance with the invention and intended for direct coupling to the crankshaft of the power press.

FIG. 2 is a section taken along the line 22 in FIG. 1.

FIG. 3 is a fragmentary section taken along the line 3-3 in FIG. 1 showing one of the pickup heads.

FIG. 3a is a phase diagram.

FIG. 4 shows an alternate form of magnetic disc for use with alined pickup heads.

FIG. 5 is a schematic diagram of one of the heads and the associated fail-safe control circuitry, the head being in the form of a proximity transducer for performing the anti-repeat function.

FIG. 6 is a schematic diagram of the remaining pickup head and its associated input circuit, the head being in the form of a velocity transducer to perform the run function.

Turning to FIGS. 1-3 of the drawings there is shown a disc and pickup or transducer assembly 10 having a frame 11 supporting a housing 12 having a removable cover plate 13. Rotatably mounted in the frame 11 is a shaft 15 having a coupling 16 at its inner end for coupling to the press crankshaft 17. The frame 11 may be supported stationarily on the press by any desired means.

At its outer end, and mounted within the housing, the shaft 15 carries a disc assembly including a disc 20 of steel or other magnetic material having separate paths including magnetic discontinuities for actuating pickup heads or transducers 21, 22. In the case of the transducer 21, which at a radius r from the disc axis, the path includes a non-magnetic opening in the form of a sector shaped cut-out 23 having a leading edge 23a and a trail ing edge 23b. The transducer 21 is secured in an adjusted position with respect to the disc by means of one or more set screws 24 threaded in a bracket 25 which is stationary with respect to the press frame. As set forth in FIG. 5, to be presently discussed, the transducer 21 is preferably constructed as a differential transformer so arranged as to be unbalanced when opposite the body of the disc for production of an output signal and balanced to produce zero signal when the transducer is opposite the non-magnetic opening or notch 23. Thus the transducer 21 is sensitive only to the proximity of magnetic material.

The transducer 22, on the other hand, is so constructed as to produce an output signal over a portion of the disc periphery and as a result of the relative movement of the disc. Accordingly the transducer 22 is mounted adjacent the path of movement of a series of discontinuities in the form of teeth 26. Conveniently, the disc may be in the form of a sprocket wheel from which the teeth have been cut or ground off over about one half of the periphery and having a pitch radius 12.

In carrying out the present invention, the output signals from the transducers 21, 22 are utilized, following passage through the circuitry to be described, to perform anti-repeat and run control functions respectively. In a typical case the anti-repeat transducer 21 is phased with respect to the disc 20 and crackshaft 17 so as to have an off time which extends over a small angle as the slide of the press approaches its uppermost position, as indicated by the phase diagram FIG. 3a. The exact duration of the off time is not important and may lie, for example, within the range of about 15 to 30. As to the transducer 22 which performs the run function, the transducer is preferably phased with respect to the disc and the crankshaft so that the transducer is on, producing output, over an angle of approximately 180, i.e., from about before bottom dead center to 10 before top dead center. The transducer is off producing zero output over the remaining 180, i.e., during the descent of the slide. As appreciated by one skilled in the art, the effect of this is to require the press operator to keep his hand on the operating pushbuttons during the period of descent. For a fuller discussion of the anti-repeat and run functions, cross reference is made to the copending application mentioned above.

In order to achieve the phase relationships set forth in FIG. 3a, the present invention contemplates bodily adjustment of at least one of the transducers and adjustment of the magnetic disc 20 with respect to the crankshaft to which it is coupled. To adjust the position of transducer 22 with respect to transducer 21 and the disc, it is preferably mounted in an arcuate slot 27 and clamped in position by a nut 28 which is threaded to the transducer body. For the purpose of changing the phase position of the disc 20 with respect to the press crankshaft 17, a shallow spline type connection 30 is provided between them. Thus there is pinned to the end of the shaft a hub member 31 having a set of external teeth 32. Surrounding the hub and registering with the teeth thereon is an annular ring gear 33 which is secured by riveting or the like to the disc 20. For the purpose of changing the phase position the disc has provision for slight axial movement against the force of a restoring spring. Thus it will be noted in FIG. 2 that the disc is urged against the hub by a coil spring 35 which surrounds the end of the shaft and which is maintained captive by a disc 36 held in place by a snap ring 37. At its opposite end the spring 35 seats against an inwardly extending flange on the disc 20. In order to visually indicate the phase adjustment between the hub 31 and disc 20, a window 40 is provided in the disc having a pointed 41 which cooperates with indicia 42 stamped in the hub. A finger hold is provided on the disc 20 in the form of a knob 45 secured to the disc by machine screws 46. The later are threaded into a cylindrical flange 47 which is integral with the disc 20. Thus to change the relative phase position of both of the transducers relative to the press crankshaft the operator pulls outward upon the knob 45 unseating the teeth 32, following which the disc 20 may be rotated into a plus or minus phase position as indicated by the pointer 41, following which the knob 45 is released to reengage the teeth 32. It may be noted that in spite of the fact that the opening 23 and teeth 26 are, in FIG. 1, arranged end to end, the two functions may be adjustably overlapped by adjusting the angular offset of the two transducers.

In accordance with one of the aspects of the invention, the disc of magnetic material, instead of being notched adjacent the teeth as in FIG. 1, may be provided with an arcuate slot which overlaps the teeth, thereby permitting use of pickup heads which are alined with one another. Thus referring to FIG. 4 a disc 20a is provided cooperating with pickup heads 21a, 22a in radial alinement. Such heads may be stationarily mounted with respect to the disc upon any suitable support. Formed in the disc for passage in front of the head 21a is a slot 23a of arcuate shape and having an angular extent which may be on the order of 40. For energizing the pickup head 22a, teeth 26a are provided over one-half of the periphery, as shown, the teeth on the opposite half being removed to provide a smooth edge. The slot 23a and the teeth 26a are preferably phased so that the slot overlaps the teeth by an angle of, say, 20-30.

For the purpose of changing the relative phase position between the disc and the paired pickup heads, the disc may be provided with a plurality of mounting holes 33:: spaced at equal intervals, for example 30, for receiving machine screws which register in tapped openings in the central driving hub which is connected to the press crankshaft. Thus the disc may be moved forwardly or backwardly through an increment which corresponds to the hole spacing. Fine adjustment within the large increment may be secured by mounting both of the heads for arcuate adjustment similarly to the mounting of the head 22 previously discussed.

Turning attention to FIG. 5 which shows the circuit employed with the proximity transducer 21 it will be convenient and will facilitate understanding to divide the circuit into separate portions or sub-assemblies. The main portion of the circuit indicated at can be considered as an amplifier having input terminals 101, 102, output terminals 103, 104-. Connected to the input terminals is an input stage 105 while the output terminals are fed. into a transformer output stage 106.

Occupying the central portion of the circuit diagram is a dual power supply 107. Energy is supplied in the form of alternating current applied to a power transformer having a primary winding 121 and secondary windings 122, 123 defining a center tap connection 124. The voltage induced in the secondary windings is rectified by diodes 125, 126, with filtering by a capacitor 127 and series resistor 128. Thus direct voltage is produced across output terminals 131, 132, terminal 132 being negative with respect to the terminal 131. The voltage is maintained at a constant value of 6.8 volts by means of a zener diode 133 which cooperates with the series resistor. In the upper leg of the circuit, and for the purpose of producing a positive output voltage, current from the secondary windings 122, 123 is rectified by diodes 135, 136 and filtered by means of a capacitor 137 and series resistor 138 feeding an output terminal 139. The voltage is held at a constant value of, say, 12 volts by means of a zener 140.

Turning attention next to the input circuit 105, means are provided for producing an A.-C. output voltage whenever magnetic material is in the proximity of the pickup head 21. The pickup head is in the form of a differential transformer having a primary winding 141 and balanced secondary winding 142 coupled to a central core 143. The differential transformer is so constructed that with no metal in the vicinity to create unbalance, the voltage induced in the winding 142 is zero. The secondary output voltage is applied to the base-emitter or input circuit of a transistor 145. Since the transformer output voltage even under unbalanced conditions, is at an extremely low level, and to avoid masking by leakage current through the transistor, negative bias is applied to the base of the transistor, counteracting the leakage. This negative bias is applied via a resistor 146 and thermistor 147 fed from the negative terminal 132 of the power supply. The resistors constitute one leg of a voltage divider, the remaining leg being formed by a resistor 148 which is returned to the positive terminal 139 of the power supply via the same line 149 which establishes the operating point of the transistor.

In accordance with the present invention means are provided for restoring proper phase to the A.-C. signal applied to the input terminals 101, 102 of the amplifier, thereby to overcome the inherent phase shift effects introduced by the pickup head 21 and which tend to vary with the speed of the press. This is accomplished by converting the signal to D.-C. and then reconverting it to square wave A.-C. by means of a transistor stage which is gated with respect to the supply line. Turning first to the means employed for converting the signal to D.-C., a transformer 151) is provided having a primary 151 and a secondary 152. A capacitor 153 in parallel with the primary winding tends to improve the signal wave shape. The resulting A.-C. voltage induced in the secondary winding is rectified by a full wave rectifier formed of diodes 161-164, with the bridge output appearing across the terminals 1&5, 166 shunted by a capacitor 167 for the purpose of smoothing out the direct voltage. This capacitor preferably has a low value, on the order of 0.25 microfarad, in order to keep the time constant reasonably short.

To reconv'ert the signal voltage to A.-C., a transistor 170 is used as a switch or gate synchronized with the A.-C. line. Thus to turn the transistor on and otf a transformer is used in the base-emitter circuit having a primary winding 171 and a secondary winding 172. For convenience the secondary winding 172 may be wound on the core of the transformer 120, the primary of which is connected to the A.-C. line. The circuit is completed by a base emitter resistor 173 and a series resistor 174. The transistor 170 is biased to produce saturation during one half cycle of the A.-C. wave, and during such half cycle the collector-emitter circuit is closed. During the remaining half cycle the transistor is cut off or open with the result that a square output wave is produced as indicated at 175, the amplitude of which depends upon the output voltage of the bridge.

For the purpose of amplifying the wave 175, a final amplifying stage 180 is provided consisting of transistors 181, 182. Resistor 183 limits the current of transistor 181 to a safe level. The transistor 181 is so biased by the resistor 184 that it is normally conducting. Thus the incoming wave 175, which is of negative polarity, is effective to make the transistor 181 periodically non-conducting Intel-posed at the input of the transistor 181 is a thermistor 185 shunted by a resistor 186 to provide temperature compensation.

The transistor 181 is, as shown, direct coupled to the input side of the transistor 182. In order to prevent flow of leakage current at high temperature, diodes 187, 188 are included in the base emitter circuit, with the diode 188 serving the additional function of providing bias for the transistor 182. The output of the transistor 182, which is still in the form of a square wave, is fed to the output terminals 103, 104 to excite the primary winding STP of a transformer 5T having secondary windings STSA and STSB. In coupling the present static switch to the control circuit disclosed in the above copending application the present secondary windings are simply substituted for the windings originally designated STSA, STSB in such application.

The above circuit has the advantage that it is positionsensitive and not velocity-sensitive, with the presence or absence of output signal depending only upon the relative phase position of the magnetic disc and the pickup head. Thus a reliable signal is applied to the control circuitry disclosed in the above copending application at all times except over a short interval just prior to the slide reaching the top of the stroke. Thus it is possible to start the press from the at rest condition. Since the disc and pickup assembly 14 is securely housed, there is no practical possibility that a piece of magnetic material of sufficient size to unbalance the differential transformer may find its way to the vicinity of the transformer core so as to produce a faulty output signal.

In accordance with one of the aspects of the present invention the run pickup head 22 is in the form of a velocity transducer capable of producing an A.-C. output signal as the teeth on the magnetic disc pass by in close proximity. The transducer input circuit 190 includes a permanently magnetic core 191 surrounded by a winding 192. The winding is included in the input circuit of a transistor 193 having a series resistor 194. Negative bias from terminal 132 of the power supply 1117 is applied by means of a voltage divider consisting of a resistor 196 and thermistor 197 in one leg and a resistor 198 in the other for the purpose of establishing the transistor operating point. The amplified A.-C. signal which exists at the output of the transistor 193 is applied to the input terminals 101, 102 of the amplifier which is the same as that previously discussed with respect to FIG. 5. Connected to the output terminals 103, 16M of the amplifier is an output circuit 200 which includes a transformer 8T for coupling the voltage to the control circuits disclosed in the copending application. The primary winding STP of the transformer is shunted by a diode 20 1 for the purpose of transient suppression. As long as the magnetic teeth pass by the pickup head 22 an A.-C. output signal is produced at a reliably high level in the secondary 8T8, but where the teeth are missing on the disc, or when the disc is stationary, the output voltage is zero.

When connecting the static run limit switch of FIG. 6 to the logic circuitry disclosed in the above mentioned patent application, the circuit disclosed in the application may be simplified as follows: The transformer 10T, its associated diodes and switches may be removed from the circuit since the checking function described in the above application is no longer required. Moreover, the secoindary windnig ZTSA of the transformer 2T and its associated diodes may be omitted. The switches in series with the transformer winding 2TP may be omitted, or bridged, so that the winding 2T]? is permanently energized. Finally, the transformer secondary winding STS which constitutes the output of the static run limit switch is simply substituted for the secondary winding originally designated 8TS in the above patent application, with the pushbutton 23 and switch contacts SS3, paralleled together, being removed from the primary circuit and connected, instead, in series with the secondary (FIG. 1). The remainder of the circuit of the copending application is incorporated herein by reference, and the resulting operating sequence, from the point of view of function, remains unchanged. One skilled in the art will appreciate, however, that the present static limit switch circuitry is not limited to use with the logic circuit of the copending application but may be employed with other control circuits intended to be operated by anti-repeat and run limit switches.

It is one of the main features of the present circuitry that it is immune to the types of failure which would normally cause a circuit to be inoperative. Thus referring to FIG. 5 it may be shown that components failure results in zero output bringing the press to a stop. Thus breakage of the leads leading from the transducer or pickup to the circuit proper terminates the signal. Also an open circuit in the transistor prevents any signal from flowing through the transformer primary winding 151. At the opposite extreme, a short circuit occurring in the transistor 145 causes a heavy flow of direct current through the transformer winding 151 thereby saturating the core and preventing any voltage from being induced in the secondary. Referring next to the bridge circuit consisting of the diodes 161-164, a short circuiting of any one of the diodes will act to short circuit the transformer winding 152 and will prevent the existence of any signal voltage at the bridge output. The individual diodes are preferably of a sufficiently high rating, as compared to the current to be conducted, that there is substantially no possibility of open circuiting. Moreover, any low resistance failure of the capacitor 167 will prevent the signal voltage from being applied to the following stage.

With regard to the switching transistor 170, open circuiting of the transistor 17d prevents passage of the signal. Conversely a short circuit in the transistor will have the effect of applying direct voltage to the following stage so that no A-C. signal can be developed across the output terminal. The same considerations apply to the transistors 181, 182 wherein open circuiting produces a break in the circuit path and short circuiting causes flow of steady D.-C. precluding the existence of an A.-C. signal. Finally, turning attention to the diodes in the power supply 107, short circuiting produces a loading effect upon the input transformer which drops the voltage below the operating level. This contingency can be taken care of, if desired, by fusing the primary supply circuit. Open circuiting of the power supply diodes individually does not substatially affect the operation of the circuit because of the large amount of filtering. Open circuiting of the diodes comprising a pair prevents the development of output voltage and thus disables the circuit. As to the remaining components, primarily resistors, these can be chosen of such rating so that the chance of failure is negligible.

All of the fail-safe considerations described above apply will equal force to the circuit of FIG. 6.

Because the control arrangement is free of moving parts subject to wear, and since all of the electrical components are static and free of relays or the like, the control arrangement may be expected to operate indefinitely, but even if component failure should occur, the failure is such as to restore the circuit to a safe condition, with the press coming automatically to a stop.

As covered in some detail in the co ending application cross referenced above, the control circuit disclosed therein is similarly free of relays or other moving elements and is similarly fail-safe upon failure of any of the electrical components. It will be seen therefore that combining the present control arrangement with that of the compending application an overall press control system is provided which is proof against the wear and hazards which are inherent in the use of electrical contacts either in switches or relays.

I claim as my invention:

1. A fail-safe static'limit switch for a power press comprising, in combination, a magnetic member rotatable with the press shaft and having a magnetic discontinuity extending over a portion of the periphery, a transducer positioned adjacent the magnetic member for producing an A.-C. signal voltage upon traversal of said magnetic discontinuity, an amplifier including a plurality of transistor stages coupled to the transducer, a transformer in the output of each of said transistor stages so that upon open circuiting or short circuiting of any of the transistors the A.-C. output of the amplifier is reduced to zero.

2. A fail-safe static limit switch for a power press comprising, in combination, a magnetic member rotatable with the press shaft and having a magnetic discontinuity extending over a portion of the periphery thereof, a transducer mounted in the vicinity of the magnetic member and so constructed and arranged that an A.-C. signal voltage is produced upon traversal of said discontinuities, means for rectifying the A.-C. signal voltage to produce a D.-C. signal voltage, an A.-C. supply line, a gate controlled by the A.-C. supply line so that the D.-C. signal voltage is converted to an AC. output wave synchronized with the voltage of the supply line and having an amplitude which depends upon the D.-C. signal voltage, and means including a transformer coupled to the gate for passing the output wave.

3. A fail-safe static limit switch for a power press comprising, in combination, a magnetic member connected for rotation to the shaft of the press and having a magentic discontinuity thereon, a transducer arranged in the vicinity of the magnetic member and so constructed and arranged as to produce an A.-C. output signal upon traversal of the discontinuity, a transistor for amplifying the A.-C. signal, a rectifier for rectifying the amplified signal, a transformer interposed between the transistor and the rectifier so that no signal is applied to the rectifier in the event of open circuiting or short circuiting of the transistor, an A.-C. supply line, means coupled to the supply line for gating the D.-C. voltage to produce a train of square waves, a power amplifier including transistors for amplifying said square waves, and an output transformer connected to the output of the power amplifier so that in the event of short circuiting or open circuiting of the transistors in said power amplifier the A.-C. signal at the output of the output transformer is reduced to zero.

4. A fail-safe static limit switch arrangement for a power press comprising, in combination, a magentic disc member connected to the shaft of the press and having teeth on a portion of the periphery thereof as well as an opening therein, transducers mounted adjacent said magnetic disc member for magnetic response to the teeth and the opening respectively, means coupled to said transducers for producing respective A.-C. output signals, means for separately rectifying said A.-C. output signals for the production of D.-C. output signals, a supply line, gates under the control of the supply line for respectively gating the D.-C. output signals to produce respective square output waves, and output means including transformers for passing said square output waves.

5. A fail-safe static limit switch arrangement for a power press comprising, in combination, a magnetic disc member rotatable with the press shaft, said magnetic disc member having teeth on a portion of the periphery thereof and having an opening therein, a first transducer cooperating with said teeth and having a permanent magnet and winding thereon for producing an A.-C. signal upon passage of the teeth and incident tothe motion thereof, a second transducer arranged adjacent the disc for response to said opening, an A.-C. supply line, said second transducer being in the form of a differential transformer having a primary winding connected to said line and a balanced secondary winding so arranged as to product an A.-C. output signal when the magnetic field thereof is unbalanced in the presence of the body of the disc and Zero output in the presence of the opening, and means for separately amplifying the A.-C. signals to produce separate A.-C. output voltages indicative of the phasing and motion of the shaft.

6. A fail-safe static limit switch arrangement according to claim 5 wherein one of said transducers is mounted for relative angular adjustment through a limited angle, the coupling means between the disc and shaft providing for register at successive angular positions so that said transducer can provide an output signal having any desired phase relation with respect to the press shaft.

7. In a static limit switch arrangement for connection to a press control circuit having input circuits for connection to an anti-repeat limit switch which is normally closed but momentarily opened during the final portion of the press cycle and a run limit switch which is closed during a substantial final portion of the press cycle, the combination comprising a magnetic disc member coupled to the press shaft for rotation therewith, said magnetic disc member having a first circular path thereon including a non-magnetic opening, a first transducer for performing anti-repeat control in the form of a differential transformer arranged adjacent the first path for producing an A.-C. output signal when the first transducer is magnetically coupled to the disc and a zero output when the first transducer is opposite said opening, said disc member having a second circular path thereon, including a series of teeth over a portion thereof, a second transducer for performing run control cooperating with the teeth in said second path and including a permanent magnet for producing an A.-C. output signal when the portion having teeth is in motion past said second transducer and a zero output when the remaining portion is opposite said second transducer, means for changing the relative phase relation between said transducers and said disc, and means for separately amplifying said output signals to produce separate output voltages indicative of the phasing and motion of the shaft.

8. In a fail-safe static limit switch arrangement for connection to a press control circuit having input circuits for connection to an anti-repeat limit switch which is normally closed but momentarily open during the final portion of the press cycle, and a run switch which is closed during a substantial final portion of the press cycle, the combination comprising a magnetic disc member coupled to the press shaft for rotation therewith, said magnetic disc member having a first circular path thereon including a non-magnetic opening, a first transducer for performing anti-repeat control in the form of a differential transformer arranged adjacent the first path for producing an A.-C. output signal when the first transducer is magnetically coupled to the disc and a zero output when the first transducer is opposite said opening, said disc member having a second circular path thereon, including a series of teeth over a portion thereof, a second transducer for performing run control cooperating with the teeth in said second path and including a permanent magnet for producing an output signal when the portion having teeth is in motion ast said second transducer and a zero output when the remaining portion is opposite said second transducer, and first and second transistor amplifiers for amplifying the respective output signals, each stage of the amplifiers having a transformer in its output so that no signal is produced in the transformer secondary winding upon a short circuit or open circuit occurring in such stage.

References Cited by the Examiner UNITED STATES PATENTS LLOYD MCCOLLUM, Primary Examiner. 

8. IN A FAIL-SAFE STATIC LIMIT SWITCH ARRANGEMENT FOR CONNECTION TO A PRESS CONTROL CIRCUIT HAVING INPUT CIRCUITS FOR CONNECTION TO AN "ANTI-REPEAT" LIMIT SWITCH WHICH IS NORMALLY CLOSED BUT MOMENTARILY OPEN DURING THE FINAL PORTION OF THE PRESS CYCLE, AND A "RUN" SWITCH WHICH IS CLOSED DURING A SUBSTANTIAL FINAL PORTION OF THE PRESS CYCLE, THE COMBINATION COMPRISING A MAGNETIC DISC MEMBER COUPLED TO THE PRESS SHAFT FOR ROTATION THEREWITH, SAID MAGNETIC DISC MEMBER HAVING A FIRST CIRCULAR PATH THEREON INCLUDING A NON-MAGNETIC OPENING, A FIRST TRANSDUCER FOR PERFORMING "ANTI-REPEAT" CONTROL IN THE FORM OF A DIFFERENTIAL TRANSFORMER ARRANGED ADJACENT THE FIRST PATH FOR PRODUCING AN A.-C. OUTPUT SIGNAL WHEN THE FIRST TRANSDUCER IS MAGNETICALLY COUPLED TO THE DISC AND A ZERO OUTPUT WHEN THE FIRST TRANSDUCER IS OPPOSITE SAID OPENING, SAID DISC MEMBER HAVING A SECOND CIRCULAR PATH THEREON, INCLUDING A SERIES OF TEETH OVER A PORTION THEREOF, A SECOND TRANSDUCER FOR PERFORMING "RUN" CONTROL COOPERATING WITH THE TEETH IN SAID SECOND PATH AND INCLUDING A PERMANENT MAGNET FOR PRODUCING AN OUTPUT SIGNAL WHEN THE PORTION HAVING 